This invention relates to heat transfer sheets and, more particularly, to a heat transfer sheet suitable for heat transfer recording by use of a printing means such as a thermal head by short-time, high-temperature heating.
As the heat transfer sheet for performing heat transfer recording in printers for computers, word processors, and facsimiles, a material comprising an ink meltable by heating applied as coating on a base sheet such as polyester film has been used heretofore. As such hot-melt ink, a mixture comprising a colorant such as carbon black, a pigment or a dye kneaded with a binder has been used.
Of the components formulated in such hot-melt ink, particularly the binder is an important component because of its great influence on various characteristics required for a heat transfer sheet such as transfer characteristic of ink and storability.
In the prior art, as such binder component, waxes or higher fatty acids have been used. More specifically, waxes such as paraffin wax, microcrystalline wax, polyethylene wax, beeswax, white wax, carunauba wax, montan wax, ceresin wax, and castor wax, stearic acid and derivatives thereof such as stearic acid, stearic acid amide, and stearic acid metal salts, and higher fatty acid amides have been employed.
However, a hot-melt ink produced by use of a binder as mentioned above involves the following problems.
(a) Generally speaking, as the characteristics required for hot-melt ink for use in heat transfer sheet, sensitivity, resolution, storability, coated film hardness, pigment dispersibility, and coating suitability may be mentioned as the main ones. In the binders of the prior art as described above, some are effective in improving several characteristics of those as mentioned above, but the above binders are not necessarily sufficiently satisfactory for improving all of the above characteristics with good balance. In other words, in the binders of the prior art as described above, there is a great tendency of the above characteristics mutually cancelling each other.
(b) In connection with the coating suitability as mentioned above, it has been difficult to use the hot-melt ink of the prior art in such a manner that two or more kinds of ink, for example, same color inks with different colors are applied as coating separately or superposed on one another on the same base sheet. The reason for this is that, when an attempt is made to apply the hot-melt ink, the ink previously coated will be melted by the heat.
For avoiding this problem, there have been attempts to use a wax emulsion or a non-aqueous system wax emulsion or to employ a special coating method, but difficulties are encountered such as poor storability of the product or low workability in coating. Thus, no practical level has been reached.
A similar problem also occurs, other than in the hot-melt ink, in the case of applying as coating an OP (over printing) layer for prevention of the so-called ground staining or for improvement of abrasion resistance.